Moonstone, Labradorite or Adularia: How to Find Your Way

Two related families, three names

Feldspars form the most widespread group of minerals in the Earth's crust. Two of them concern us here.

• The “classic” moonstone is an alkali feldspar (composed of sodium and potassium), with the general formula (Na,K)AlSi3O8, most often orthoclase. It shows a milky, bluish reflection that seems to float inside the stone: adularescence. It appears because the crystal is made of fine alternating layers of two feldspars (orthoclase and albite), which diffract light. The thinner the layers, the more the reflection tends towards blue.
• Adularia is the old name for the clear feldspar variety from which moonstone was originally obtained. It takes its name from the Adula massif in the Swiss Alps. In everyday language, “adularia” and “moonstone” refer to the same family.
• Labradorite is a plagioclase, a feldspar rich in calcium (50 to 70% calcium end-member). It shows labradorescence: patches that light up in blue, green, yellow or orange depending on the angle, then go dark grey. The type locality is Paul Island, near Nain, in Labrador (Canada); it is also mined in Madagascar, Finland, Norway and elsewhere. A highly coloured Finnish variety, discovered at Ylämaa in 1940, is called spectrolite.

For moonstone, the main sources are Sri Lanka (which supplies most of today's market), India, Madagascar, Myanmar and Tanzania. Both stones have a hardness of 6 to 6.5 and two cleavage planes: a knock can split them.

Rainbow moonstone: what are we talking about?

The trade uses “rainbow moonstone” for a white or colourless, very clear labradorite that shows several colours. It is not an alkali feldspar like original moonstone, but a plagioclase. It is a very widespread trade name, which many mineralogists consider inaccurate. It is not a fraud if it is explained: the stone is natural and real. The problem arises when a label leads buyers to believe it is the same stone as the blue-sheened moonstone from Sri Lanka.

There are also moonstones with various body colours (white, grey, peach, green), whose tint comes from impurities or inclusions and is in no way artificial.

Adularescence or labradorescence: the table

These markers are tendencies: some glasses are designed to imitate the reflection, and some poor-quality moonstones are pale. The most telling features are the movement of the reflection and the presence of round bubbles, or the total uniformity of an imitation.

Simple, safe tests

1. Light the stone with a single lamp against a dark background and tilt it slowly. The reflection of real moonstone glides and changes intensity; that of a glass stays frozen.
2. Look at the colour of the effect. A milky blue sheen suggests adularia; localised multicoloured flashes suggest labradorite.
3. Observe under a loupe: round bubbles or a completely uniform appearance point to glass; small fractures shaped like flakes or insect legs are more likely natural.
4. Look through the stone. A natural moonstone stays greyish-white when backlit; an opalescent glass often takes on an orange tint, while it looks bluish in reflected light.
5. Compare the weight with a known stone: glass is a little lighter, plastic much lighter.

Avoid: acid, flame, solvent, scratching and knocks. Feldspars split along their cleavage planes, and a scratch test would leave a mark. What these tests cannot prove: they cannot distinguish natural moonstone from a treated piece or from another feldspar with reflections. For a valuable piece, a laboratory certificate is necessary.

Imitations, treatments and misleading names

The most common imitations are opalescent glasses, sold under names such as “opalite”, “synthetic moonstone” or “glass moonstone”. The word “opalite” causes confusion: it sometimes refers to a manufactured glass and, more rarely, to a variety of common opal. Opalescent glass is neither moonstone, nor opal, nor quartz. Natural moonstones and labradorites rarely involve treatments, but a piece may be waxed, oiled or coloured on the surface: ask.

Another pitfall: very dark labradorite, almost black, which shows reflections only when turned, does not indicate a flaw. This is the normal behaviour of labradorescence. Conversely, a stone that sparkles from every side at once, with colours like soap bubbles, is more likely to be a coating or glass.

Care and choice

These feldspars are hard but cleavable. Avoid knocks, ultrasonic cleaners, household products and sudden temperature changes. A soft cloth and lukewarm water are enough. Store them apart from harder stones, which could scratch them.

When choosing: ask for the exact nature (“moonstone” in the strict sense, “labradorite”, “rainbow moonstone” = labradorite), the stated country of origin and any disclosed treatment. The profiles for moonstone and labradorite cover their symbolism; how to read a label explains the terms used.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Recognizing real stones.